Did you know that you can get tattoos that glow under UV light? The tattoos are invisible in normal light (minus the possible scarring from the tattoo procedure itself). They've been around for a decade apparently. Over the years different special black light reactive inks have appeared on the market. (Here's the Wiki on it.) I don't know how safe this stuff is, whether it's carcinogenic or allergenic or not. For a while, people were using Everglow, which is a boat paint not meant for use on human skin.
People get dragons, glowing jellyfish, spaceships, skulls, bones. They sometimes get the UV tattoo over preexisting designs for a little extra dimension.
If you need to hide your tats for a job, or if you spend lots of time in black light, or feel like emulating a radioactive superhero, or are just looking for a way to spice up your tramp stamp, this may be the thing for you.